This invention relates to ceramic capacitors having a medium temperature firing dielectric of lead-lanthanum zirconate-titanate and glass, and more particularly to such capacitors wherein the ceramic is doped and reacted with silver. The term doped as used herein is intended to mean reacted and not simply mixed with.
Glass containing ceramic dielectric materials are known as being capable of densification at medium firing temperatures, whereas ceramics without glass generally require a firing temperature of greater than 2200.degree. and often at least 2450.degree. F.
Lead-zirconate-titanate ceramics have been employed for many years as electromechanical transducers and more recently lead-lanthanum zirconate-titanates have found use in electro-optic transducers. For these purposes, the ceramic is formulated so as to form a ferroelectric body, which formulations are unsuitable for making electrical capacitors that must exhibit a relatively stable dielectric constant as a function of the applied electric field. Furthermore, these bodies tend to undergo a substantial out diffusion of lead at firing which must be so compensated that the proper composition after firing to maturity is assured. The potential loss of the poisonous lead to the atmosphere is a hazard to the human operators, is a corrupting influence in the manufacturing equipment and is a source of much difficulty in maintaining a uniform composition of the fired bodies.
It is often desirable to have a capacitor that exhibits a low temperature coefficient of capacitance (TCC), a low voltage coefficient of capacitance (VCC) and a high insulation resistance. It is further desirable that the dielectric material have a high dielectric constant to minimize the required size of the capacitor of a given capacity value.
It is therefore an object of this invention to provide an improved ceramic capacitor that exhibits a low VCC, a low TCC, and a high insulation resistance.
It is a further object of this invention to provide a ceramic capacitor dielectric material that is capable of being fired at temperatures below 2000.degree. F. and that exhibits a low VCC and dielectric constant greater than 300.
It is a further object of this invention to provide a low cost monolithic capacitor suitable for use at high a.c. and d.c. voltages.
It is another object of this invention to provide a lead-lanthanum zirconate-titanate dielectric body by a method wherein only small amounts of lead are lost at firing and having predictable performance characteristics.
It is yet another object of this invention to provide a lead-lanthanum zirconate-titanate dielectric material that does not require special firing atmospheres.